Polar Cap Magnetic %riationsand Their Relationship,with theInterplanetary Magnetic Sector

Structure

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by

Leif Svalgaard

Reproduction in whole or in partis permitted for any purpose ofthe United States Government.

September 1972

SU-IPR Report No. 486

Prepared under

Office of Naval Research Contract N00014-67-A-0112-0066

National Aeronautics and Space Administration

Grant NGR 05-020-559N , ••

National Science Foundation Grant GA-31138

INSTITUTE FOR PLASMA RESEARCHSTANFORD UNIVERSITY, STMFORD, CALIFQRNlA

POLAR CAP MAGNETIC VARIATIONSAND THEIR RELATIONSHIP WITH THE

INTERPLANETARY MAGNETIC SECTOR STRUCTURE

by

Leif Svalgaard

Office of Naval ResearchContract N00014-67-A-0112-0068

National Aeronautics and Space AdministrationGrant NCR 05-020-559

National Science FoundationGrant GA-31138

Reproduction in whole or in partis permitted for any purpose ofthe United States Government.

SUIPR Report No. 486

September 1972

Institute for Plasma ResearchStanford UniversityStanford, California

IPolar Cap Magnetic Variations and Their Relationship

with the Interplanetary Magnetic Sector Structure

Leif Svalgaard

Institute for Plasma Research, Stanford University

Stanford, California 94305

Abstract

The relationship between polar geomagnetic variations and thei

polarity of the interplanetary magnetic sectors has been studied for the

quiet year 1965. . It is found that during the day-hours a system of

ionospheric currents encircles the magnetic poles on every day. The

current system may extend up to 15 from the pole, but is strongest at

8-10 invariant colatitude. The current direction as seen by an observer

' standing on the earth is counterclockwise during interplanetary sectorsi

r with field pointing away from the sun, and clockwise during toward sec-

tors. The current strength is dependent on season, being strongest

during local summer. When the magnetic pole is on the nightside of the

earth, this polar cap current is absent or very weak. When the rotation

of the earth brings the magnetic pole into the dayside, the polar cap

current system develops, with the current being most concentrated in the

part of the current system which is nearest to the noon meridian. The

current increases its total intensity until the magnetic pole is rotated

past the noon meridian; then the intensity decreases as the magnetic

pole approaches the nightside again. The seasonal variation of the mag-

netic elements in the polar cap is discussed in view of the sector

2.

polarity effects. These effects introduce an important modulation of

the seasonal variations of the geomagnetic polar field. The demonstra-

tion of current systems inside the polar caps encircling the magnetic

poles during local day hours calls for a major revision of the generally

accepted picture of polar cap geomagnetic variations. It also suggests

a new framework for interpreting polar cap observations of geomagnetic

and related phenomena.

3.

Polar Cap Magnetic Variations and Their Relationship

with the Interplanetary Magnetic Sector Structure

Leif Svalgaard

Institute for Plasma Research, Stanford University

Stanford, California 94305

Introduction

The diurnal variation of the geomagnetic field components in

the polar caps is traditionally considered to be simple, consisting of a

rather regular sinusoidal wave in all three components. The variation

of the horizontal components resembles the magnetic effects of a uniform

horizontal current sheet covering the entire polar cap. This current

sheet stays fixed in relation to the direction to the sun, while the

earth rotates under it. Figure l(a) shows the diurnal variation during

the summer season of the North component X and of the East component Y

at Resolute Bay near the northern magnetic pole. The form of the two

curves is sinusoidal to within a few gammas, and their phases differ by

six hours. The uniformity of the variation across the polar cap is

illustrated by Figures l(b) and l(c) for four observatories in the cen-

tral northern polar cap: Alert, Thule, Resolute Bay, and Mould Bay.

Figure l(b) shows that the time of maximum X is controlled by local

time, i.e., the rotation of the earth, so that the current sheet stays

fixed. The amplitude of the regular diurnal variation is very nearly

constant over the polar cap as shown in Figure l(c), being larger during

the summer than in the winter. Also the amplitude on disturbed days is

4.

far larger than on quiet days.

Since the westward auroral electrojet is nearest to a polar

cap station in the early morning hours, an increase of the vertical com-

ponent Z is then observed. In the late afternoon, the eastward auroral

electrojet is nearest to the station and a depression of Z is observed.

The variation of Z during a day is thus also of a regular character.

The amplitude of this variation increases with increasing distance from

the magnetic pole, since this brings the observing station close to the

electrojets.

It is generally believed that some of the currents which close

the auroral electrojets are the cause of the uniform perturbation field

across the polar cap, and therefore the cause of the regular daily vari-

ation of the polar field. By way of the electrojets, geomagnetic dis-

turbances within the polar caps are tied to the substorm activity in the

auroral zones. If the substorm activity is high, strong disturbances

are always observed inside the polar cap. On the other hand, consider-

able magnetic disturbance may be observed in the polar cap even if the

activity at lower latitudes is very low, showing that processes parti-

cular to very high latitudes are in effect.

The special polar cap disturbances differ from auroral lati-

tude activity in several important ways, the most prominent being that

auroral latitude activity peaks at midnight while polar cap disturbances

are most prominent around noon. The polar cap disturbances are usually

described as being irregular without distinct patterns, so that they

cancel out when several days are averaged; in this way the average daily

variation takes on its simple form as described above. However,

5.

Svalgaard (1968, 1972) and independently Mansurov (1969) found a regu-

larity in the polar cap disturbances related to the direction of the

interplanetary magnetic field. When the earth is immersed in an inter-

planetary magnetic sector (Wilcox, 1968) with field directed away from

the sun, daytime disturbances are observed at very high latitudes which

are directed dominantly away from the earth. When the earth is within

a toward sector, the disturbance field is directed toward the earth in

both polar caps. A recent review of this effect has been given by

Wilcox (1972).

This relationship between the interplanetary magnetic field

(IMF) polarity and the daytime polar cap disturbances is so distinct

even on individual days that it has been possible to infer the IMF

polarity on a day-by-day basis (Friis-Christensen et al., 1971; Svalgaard,

1972). A closer inspection of the few days of disagreement between the

inferred field polarity and the field polarity observed with spacecraft

showed (Friis-Christensen et al., 1972) that on most of such days the

direction of the interplanetary field departed considerably from the

average Archimedean spiral direction, causing the azimuthal component

of the field to be in the opposite direction to that expected for a

spiral-like field. This led Friis-Christensen et al. (1972) to suggest

that the cause of the effects observed in the polar geomagnetic field was

not the IMF polarity as such, but rather the direction of the azimuthal

component of IMF. It is, however, relatively rare that the IMF deviates

so much from the spiral direction that the sign of the IMF polarity and

the sign of its azimuthal component differ.

In this paper we shall examine the magnetic disturbance pat-

6.

terns in the polar cap observed on days with the same IMF polarity. We

will show that a circulating ionospheric Hall current flowing at about

82 invariant latitude eastward around the northern magnetic pole ex-

plains the observed perturbation when the earth is within an away sector.

When the earth is immersed in a toward sector, a westward current of

about the same strength and location fits the observations.

Since it is possible to infer the IMF polarity on almost

every day, we are led to the following explanation of the observed high

latitude geomagnetic disturbances: during the day-hours a system of

ionospheric Hall currents encircles the magnetic poles on every day.

The current system may extend up to 15 from the pole, being strongest

at 8-10 invariant colatitude. The current direction as seen from the

earth is counterclockwise in away sectors and clockwise in toward sec-

tors. Finally, the current strength is dependent on season, being strong

during local summer and weak during the winter.

The existence of a current system inside the polar cap encir-

cling the magnetic pole during local day-hours calls for a major revision

of the currently accepted picture of polar cap geomagnetic variations.

It also suggests a new framework for interpreting polar cap observations

of geomagnetic and related phenomena.

Data Analysis

The present study uses geomagnetic data supplied by World Data

Center A for Geomagnetism, as well as sector polarity data given by

Wilcox (1968). The analysis extends over the year 1965, partly due to

availability of digitized data for that interval. To eliminate days on

7.

which the IMF was not close to the spiral direction or when the field

was of mixed polarity the following procedure was used. Using Z-magneto-

grams from Resolute Bay and from Thule the IMF polarity was inferred

independently for each day of 1965 whenever possible. For details about

the method, see Svalgaard (1972). Only days on which the IMF polarity

inferred from both stations and the polarity measured by spacecraft all

agreed were then selected. Table 1 shows the measured IMF polarities,

together with the inferred polarities.

Hourly mean values of the geomagnetic field components for nine

stations in the northern hemisphere were used in the analysis. This in-

cluded four stations in the central polar cap — Alert, Thule, Resolute

Bay and Mould Bay — three auroral zone stations — Point Barrow, Fort

Churchill and Leirvogur — and two stations at intermediate latitudes —

Godhavn and Baker Lake, which provided additional coverage. The geomag-

netic field data was first converted to X, Y and Z components and then

corrected for secular variation. All days with data gaps were then ex-

cluded to allow meaningful daily variations to be computed.

Daily variations of all three components were then computed for

each IMF polarity separately for the whole year, as well as for different

seasons — a season being here defined as 120 days centered on a solstice

or an equinox. Finally, the average daily variations through the year

were computed using all days irrespective of IMF polarity. Since the com-

puted daily variations consist of sets of 24 hourly field values instead

of deviations from the da'ily mean" value, they may be directly compared

with each other. The uniformity of the average daily variation of the

polar cap field as discussed in the introduction indicates that variations

8.

Table 1

IMFJ 01 Rlt

TH

IMFF 24 RD

TH

IMFM 23 RB

TH

IMFA 19 RB

TH

IMFM 16 RB

TH

IMFA OS RB

TH

IMFS 01 RB

TH

IMFS 28 RB

TH

IMFO 25 RB

TH

IMFN 21 RB

TH

IMF0 18 RB

TH

IMF + +J 28 RB + +

TH + -

IMF + + +J 12 RB +

TH + + -

IMFJ 09 RB -

TH + +

Comparison between measured IMF polarity and the polarity inferredusing the polar cap observations Resolute Bay (RB) and Thule (TH).The data are ordered in 27-day periods with the starting date ofeach period given to the left of each period. A plus (+) is thesignature for a day with positive (away) polarity. A minus (-) isthe signature for a day with negative (toward) polarity, while ablank indicates a day where the polarity is unknown or badly defined.The days are divided into groups of seven. Note that on several daysthe polarities inferred using both RB and TH mutually agree but dis-agree with the IMF polarity measured by spacecraft. All such dayswere excluded from the analysis.

9.

with period of several hours are not influenced very much by local

inhomogenities in subsoil conductivity. This is especially encouraging

in the case of Alert, which shows severe induction effects for short

period variations.

Figure 2 shows the diurnal variation of the Z-component at

Resolute Bay; the open circles display the variation found on days where

the earth is within away sectors, while the filled circles show the var-

iation during toward sectors. For about half of the day the sector

polarity does not seem to have any influence on the value of the Z-com-

ponent. However, during the other half of the day, roughly between 12

and 24 UT, the variation of the Z-cbmponent is strongly dependent on

the IMF polarity. On days with positive IMF polarity (away from the sun)

the Z-component is decreased and reaches a minimum around 18 UT, while

on days with negative polarity the Z-component is increased by about the

same amount for about the same interval of time as the decrease on days

having positive polarity.

The average daily variation found for all days of the year

is shown by the broken curve in Figure 2, demonstrating the almost com-

plete cancellation of the sector polarity influences when about the same

number of days with opposite IMF polarity is averaged together. This

result suggests that the same mechanism is responsible for both the in-

crease of the Z-component during toward sectors and for the decrease

during away sectors; the sign of the change in the Z-component being

determined only by the sector polarity or more strictly by the sign of

the azimuthal component of the interplanetary magnetic field. When the

average daily variation is subtracted from the data, only the sector

10.

polarity influence on the Z-component remains and this is shown in the

lower part of Figure 2.

We have discussed this figure at some length because the in-

fluence of IMF polarity on the vertical component at Resolute Bay is

rather typical for other polar cap stations and for other magnetic ele-

ments as well. For any given polarity of the IMF we find a certain

V* V\

deviation (taking place roughly between 12 and 24 UT) from the simple

all-year-average daily variation of each magnetic element. For the

opposite polarity about the same deviation is observed, but with the

opposite sign. Figure 3 shows another example of this general behavior

for the North component at Resolute Bay. In this figure the broken

curve showing the average diurnal variation (for all days irrespective

of the IMF polarity) has been slightly smoothed to approximate the simple

sinusoidal variation.

The same smoothing procedure has been applied to all average

daily variation data used in this study. The effects of the smoothing

are in all cases rather minor, never exceeding 5 gammas. Since this is

but a small fraction of the total variations no significant bias is

introduced by the smoothing.

The daily variations for the two IMF polarities taken separ-

ately are based on about 80 days of each polarity, while the yearly

average daily variations shown by the broken curves are based on most

of the days through the year (usually about 350 days excluding those

with data gaps).

The results of the present analysis rest on the assumption

that the regular nearly sinusoidal diurnal variation of the magnetic

11.

elements described in the introduction and approximately given by the

slightly smoothed all-year average variation is a real phenomenon, prob-

ably caused by activity outside the central polar caps. The influence

of the sector polarity is then thought to be superposed on the simple

diurnal variation, and may be extracted by subtracting this variation

from the observed diurnal variation of a given element for a given polar-

ity. Examples of the result of the subtraction are given in the lower

part of Figures 2 and 3. The assumption of superposition of the simple

diurnal variation and the IMF polarity effects may be justified by

examining magnetograms for individual days when the polarity effects are

of unusually short duration; in these cases the simple diurnal variation

can be followed up to the onset of the polarity effects and it continues

with the same trend after the effects have ended.

Polar Cap Geomagnetic Response to IMF Polarity

Using the data and procedures discussed in the previous sec-

tion the nature of the influence of the polarity of IMF on the high

latitude geomagnetic field has been studied in detail. The polarity

effects over the northern polar cap were found to be strongest at 17 -

18 UT. At that time it is local noon at the northern magnetic pole,

and conditions are the most favourable for direct interactions betweent\

the solar wind and the polar cap geomagnetic field. At stations in

intermediate latitudes .(about 75 ) the polarity effects-are still clear-

ly discernible, whereas there is only very slight indication of any IMF

polarity influence at auroral latitudes (below 70 ). The effects are

12.

thus well confined to the interior of the polar cap and cannot be inter-

preted as extensions of certain types of auroral latitude activity.

A synoptic presentation of the superposed disturbances ob-

served at 18 UT during away and toward IMF polarity sectors is given in

Figures 4 and 5 respectively. Horizontal perturbations are shown as

vectors attached to station circles for 6 stations in the northern polar

cap. The simultaneous vertical perturbation is given as a signed number

next to the station circle. The positions of the geographical pole (GP)

as well as of the magnetic pole (MP) are indicated on the figures.

A word should be said about the definition of the sign of the

vertical component. Traditionally the Z-component is considered posi-

tive when directed downwards, i.e., towards the earth. Hence Z is posi-

tive in the northern hemisphere and negative in the southern hemisphere.

A perturbation directed away from the earth is then negative in the

northern polar cap and positive in the southern polar cap.

Analysis of Figures 4 and 5 reveals a clear systematic differ-

ence in the way the geomagnetic field is disturbed during conditions

with opposite IMF polarities. For away IMF polarity (Figure 4) the

horizontal perturbation vectors all converge towards the magnetic pole,

and vertical perturbations directed away from the earth (negative in

northern hemisphere) are observed near the pole while vertical perturba-

tions towards the earth (positive in northern hemisphere) are seen below

o80 invariant latitude. For towards polarity (Figure 5) the direction

of all perturbations is reversed; horizontal perturbations diverge from

the magnetic pole and vertical perturbations towards the earth occur

near the pole.

13..

These magnetic effects are precisely what might be produced

by a circulating ionospheric current flowing eastward around the mag-

netic pole during away sectors, and flowing westward during toward sec-

tors. The direction of the current around the southern pole is opposite

to that of the northern since the perturbation of the Z-component has

opposite signs in the two hemispheres (Svalgaard, 1972). For an obser-

ver standing on the earth the current direction would be clockwise for

toward IMF polarity and counterclockwise for away polarity in both hemi-

spheres. The magnitude of the effect, however, seems to be largely in-

dependent of the polarity of the interplanetary magnetic field. There

are indications (Friis-Christensen et al., 1972) that the magnitude of

the effect is proportional to the magnitude of the azimuthal component

of the IMF, with the constant of proportionality depending on season,

being largest during local summer.

From an analysis of geomagnetic data from seven stations in

the northern and eleven stations in the southern hemisphere, Mansurov

and Mansurova (1971a) deduced essentially the same current systems to

account for the polarity effects observed within the polar caps. The

present study fully confirms their results, which apparently have

attracted little attention. Studying the polarity effects on single

days, Friis-Christensen (1971) also concludes that the magnetic pertur-

bations may be due to a current vortex with focus near Thule at about

I7h UT.

The magnetic effects of the zonal current systems described

above have the same sign throughout the day for given stations and IMF

polarity. This means that daily mean values of a given element should,

14.

show systematic differences between days within IMF sectors with differ-

ent polarity. Furthermore by taking the mean value over a day field,

variations of the simple sinusoidal type cancel out to enhance the

superposed polarity effects. This allows us to make a simple straight-

forward analysis of the influence of different IMF polarity on the geo-

magnetic field.

The average values of the three components (X, Y, Z) were com-

puted for all nine stations for three samples of the data through 1965;

namely (i) for all days within away sectors, (ii) for all days within

towards sectors, and (iii) for all days irrespective of the sector

polarity. Subtracting the average field values for the last sample from

the first samples should then show any effects related to IMF polarity.

Figure 6 shows the result plotted against invariant latitude. The IMF

polarity effect in the Z-component is again characteristic of a current

encircling the magnetic pole; the current direction again reverses when

the IMF polarity reverses. The Z perturbation changes sign at about 80

invariant latitude suggesting that the current is most concentrated near

this latitude. The magnitude of perturbations of the horizontal com-

ponent across the polar cap is shown in the lower part of Figure 6. The

horizontal effects are strongest at about 82° invariant latitude.

We should expect the vertical perturbation to change sign at a

slightly lower latitude than that where the horizontal effects are

strongest, because the magnetic effect of the part of the current system

at some distance from 82 is mainly in the vertical direction. The im-

portant thing here is that a simple and straightforward analysis using

mean values over many days leads to the same conclusion as the analysis

15.

based on the somewhat more subtle diurnal variation of the magnetic

elements, namely that the IMF polarity effect on the geomagnetic field

can be described as the magnetic effects of a current system encircling

the magnetic poles at very high latitudes only. The current direction

changes when the polarity of the interplanetary magnetic field changes.

Temporal Changes in the Polar Cap Current

If the polar cap current system around the magnetic poles is

stationary in position the direction of the magnetic perturbation vec-

tors caused by it should not change during the day. The immediate im-

pression from the analysis of the daily variation of the perturbations

is that their direction indeed stays relatively constant throughout the

interval in which they are observed. To examine this more quantitative-

ly the horizontal perturbation vectors for Thule, Resolute Bay and Mould

Bay are presented in Figure 7. It is evident from the figure that the

direction changes observed during the twelve-hour interval in question

V> Vi(12 - 24 UT) are in fact rather small; the curve described by the

endpoint of the vector is generally elongated either towards the magnetic

pole or away from it. This indicates that the current system does not

move very much during the day. On the other hand the data are sugges-

tive of a slight displacement towards the geographical pole of the

current during the interval in which the effects of the current are

observed.

The perturbation vector rotates clockwise at Thule and counter-

clockwise at Mould Bay, irrespective of the sense of the current (or of

16.

the IMF polarity), while Resolute Bay shows a transitional behavior in

the sense that the vector rotates clockwise during away sectors and

counterclockwise during toward sectors. Since Mould Bay is located

roughly to the west of the magnetic pole, while Thule is located to the

east of the magnetic pole and Resolute Bay is to the south, a slight

south-to-north movement of the current system during the day accounts

for the vector rotations observed. The data from Resolute Bay are the

most sensitive ones to variations of the direction of movement but with

no vector rotation on the average. By noting that the azimuthal change

in the perturbation vectors is from 30 to 50 for stations 4 to 8

from the pole, we can estimate the total displacement of the current

system focus to be about 3° of latitude or 300 km, crossing the magnetic

pole from south to north. Compared to the 15 - 20 diameter of the

current system, this displacement is but a minor secondary effect. The

main conclusion is that the current system occupies the central polar

cap around the magnetic pole throughout its existence.

While the location of the current system does not change very

much, the magnitude of its magnetic effects (and therefore probably the

current intensity) on the contrary shows a very marked variation

throughout the day. This is easily seen in Figures 2, 3 and 7 where the

largest perturbations are observed in the interval 15 - 21 UT while

V^ T_

the amplitude has decreased to zero in the interval 3 - 9 UT. Table 2

gives the time of maximum perturbation of the vertical component for

four stations in the northern polar cap. As the variation of the Z-

component is the integrated effects of a large section of the current

system, this component is particularly well-suited to show large scale

17.

Toward

14̂ 5 UT

15.5

17.5

19.0

Away

16hO UT

16.5

18.3

19.5

Noon

16̂ 2 UT

16.6

18.3

20.0

variations of the current intensity. The average time of maximum per-

turbation is about 17 UT. This is close to the time of local noon over

the northern magnetic pole.

Table 2

Station

Alert

Thule

Resolute Bay

Mould Bay

UT time of maximum perturbation of the verticalcomponent for towards and away IMF polarity.The time of local noon is given for comparison.

Despite the fact that there exists a local magnetic time dif-

ference of nearly fifteen hours between Alert and Mould Bay, the time

differences between maximum perturbation at different stations are much

smaller and seem to correspond to the local time differences. It seems

fair to conclude that .the largest perturbations due to the polar cap

current system are observed near local noon at all four stations.

The following interpretation of the data might be suggested.

When the magnetic pole is on the nightside of the earth, the polar cap

current is absent or very weak. When the rotation of the earth brings

the magnetic pole into the dayside,'the polar cap current system

develops, the _current.. being..most .concentrated in the part of the current—

system which is nearest the noon meridian. The current increases its

total intensity until the magnetic pole is rotated past the noon meridian;

18.

then the intensity starts to decrease as the magnetic pole approaches

the nightside again. The initial development of the current system seems

to be strongest slightly south of the magnetic pole; the current density

is then increasing northward during the day-hours.

There is an interesting asymmetry between the two IMF polar-

ities with regard to the time of maximum development of the northern

polar cap current system. The current seems to reach its maximum inten-

sity about one hour earlier during toward IMF polarity than during away

polarity. This effect is clearly seen in Table 2 as well as in Figure 7.

It is not known if the same asymmetry is present in the southern polar

cap or if it goes in the opposite direction. The fact that the polar

cap current develops on the dayside, and also the finding of

Friis-Christensen et al. (1972) that the azimuthal component of IMF is

the critical component in determining the sense of the polar cap current -

indicate that the mechanism responsible for the current works most

effectively in the front of the magnetosphere near the cusp region. A

slight displacement of the cusp region in the direction of the azimuthal

component of IMF could be the cause of the asymmetry in the maximum

development of the polar cap current for different IMF polarities. There

are indications (Friis-Christensen, 1971) that the asymmetry may be as

large as three hours during sunspot maximum years, while the one hour

difference found in the present study refers to sunspot minimum.

Seasonal Variations of the Polar Cap Current

The magnitude of the influence of the IMF polarity on the

polar geomagnetic field exhibits a very clear seasonal variation, being

19.

largest during local summer. Since the effects are almost equal in

magnitude but with opposite sign for opposite IMF polarities, and since

the effects last about half a day, a simple measure of the magnitude of

the polar cap perturbations for a given season would be the difference

between the seasonal average of the Z-component during toward sectors

and the average for away sectors for the same season. By taking the

difference, AZ, we double the effect, but by taking averages over whole

days instead of the interval where the effects are seen, we halve the

doubled magnitude again. The seasonal variation of AZ for three polar

cap stations is shown in Figure 8. The magnitude of the effect is very

small during winter but increases sharply towards the summer. In addi-

tion we note that the change throughout the year is nearly identical for

all three stations; this seems to be a consequence of the fact that all

three are well within the rather uniform perturbation field of the en-

circling polar cap current.

The very pronounced seasonal variation of the intensity of the

polar cap current may be an indication of dependency on ionospheric con-

ductivity or on the tilt of the magnetic axis of the earth, or on both.

There exists another seasonal effect in the time of maximum

perturbation, which for the northern polar cap stations studied occurs

earlier during the summer than during the winter. Table 3 lists the

time of maximum perturbation for five stations during different seasons.

In this table no distinction has been made between the two IMF polar-

ities, because the-time difference between maximum perturbation for the

two polarities taken separately is small and constant. There is a pro-

gressive change towards earlier hours of maximum perturbation as we go

20.

from winter to summer. This effect might depend on geographical lati-

tude; it is largest at the northernmost station, Alert, and becomes

smaller with decreasing geographic latitude. There is evidence

(Figure 9) from Vostok (invariant latitude -84.9 ) that this seasonal

effect is reversed in the southern polar cap, so that the time of maxi-

mum perturbation changes towards later hours from winter towards local

summer. However, more study of this particular effect is needed before

a definitive conclusion can be reached on that point.

Table 3

Winter Equinox Summer Change Latitude

18̂ 0 UT

18.0

21.5

18.5

17.0

14^0 UT

15.5

19.5

18.0

16.5

13.0 UT

15.0

19.0

17.5

16.5

5hO

3.0

2.5

1.0

0.5

82?5

77.5

76.2

74.7

69.2

Station

Alert

Thule

Mould Bay

Resolute Bay

Godhavn

Seasonal variation of time of maximum perturbation of polar cap geomag-netic field. Average time for both polarities of IMF is given becausethere is no systematic difference between the two polarities with regardto the change to earlier hours of the perturbations during summer. Thetotal change from winter to summer is also given together with the geo-graphic latitudes of the stations.

We have shown that significant daily and seasonal variations

of the location and intensity of the polar cap current system exist;

their nature suggests that ionospheric conductivity as well as magneto-

spheric geometry probably both play a role for the mechanism responsible

for originating and maintaining the polar cap current system. The vari-

ation of ionospheric conductivity across the polar cap is so small that

21.

the small-scale variations of the morphology of the current system

(such as an earlier development of the system during toward sectors)

cannot be explained by differences in conductivity.

Seasonal Variation of Magnetic Elements in the Polar Cap

It has been suggested by Nishida et al. (1966) that the vary-

ing shape of the magnetosphere during the year - due to the varying

angle between the average solar wind velocity and the geomagnetic dipole

axis — should result in a seasonal variation in the vertical component

of the polar magnetic field. Although a simple application of the mech-

anism proposed by Nishida et.al. would predict much larger variations in

the horizontal component (of the order AH == ZAZ/H) than in the vertical

component, as to render their explanation dubious, it has been shown by

a number of authors (Mansurov and Mansurova, 1971b, and references given

there) that systematic seasonal variations of (for instance) monthly

mean values of the magnetic elements do exist indeed even at high lati-

tudes. In summary, the magnitude of the vertical component is decreased

during local summer and the horizontal component directed towards the

geographical pole is increased. This effect was confirmed from the data

used in the present analysis and the results are illustrated in Figure

10. The cause of this variation is unknown, but it is probably of mag-

netospheric origin, related in some 'way to the varying geometry of the

interaction between the solar wind and the geomagnetic field.

The magnetic effects of the polar cap current system discussed

in this paper introduce an important modulation of the seasonal variation

22.

of the magnetic elements. If there is a greater number of days with one

polarity than with the other during any given season, then the mean

value of the field components for that season would be contaminated by

the IMF polarity effects. Mansurov and Mansurova (1971b) have commented

on this problem and explained why the seasonal variation of the field in

some years is simple and regular while in other years it is irregular

and distorted. This is simply the effect of variations in the propor-

tion of one IMF polarity to the other. A regular variation of this

kind has been suggested by Rosenberg and Coleman (1969), namely that the

predominant polarity of the IMF observed near the earth has an annual

variation that changes sign when the heliographic latitude of the earth

changes sign. Using the 45 years of interplanetary field polarity in-

ferred by Svalgaard (1972), Wilcox and Scherrer (1972) were able to give

a quantitative confirmation of this annual variation. There are also

indications of a complex sunspot cycle variation of the predominant IMF

polarity (see the review by Wilcox, 1972). We should therefore expect

corresponding modulations of the seasonal variation of the polar geomag-

netic field.

Figure 11 presents a summary of the seasonal variations of the

three field components as observed during 1965 for several northern

polar stations for both IMF polarities separately as well as the average

variation irrespective of sector polarity. As in Figure 2, open circles

show the variation during IMF away polarity, solid circles during toward

polarity, and broken curves show the average variation. One may note in

particular that the seasonal variation of the Z-component comes out very

small during toward polarity while it is enhanced during away polarity.

23.

Interpreting the seasonal variation of the geomagnetic elements shown by

the broken curves as the combined result of a basic rather uniform vari-

ation and of persistent magnetic perturbations being largest during the

summer season, as shown in Figures 12 and 13 separately for both IMF

polarities — provides a satisfactory explanation of the observed and

sometimes complex variations summarized in Figure 11.

In preparing Figure 11, no days have been excluded on the

ground that the inferred IMF polarity disagreed with that observed

in space. In this way more data could go into the figure, and at the

same time we get a check on whether any artifacts have been introduced

by the exclusion. The main effect of using all data is a somewhat re-

duced amplitude of the magnetic perturbations, which one would expect

due to spurious deviations from the ideal spiral direction of the IMF;

but apart from this, no significant influence on the results could be

detected.

The perturbations shown in Figures 12 and 13 correspond close-

ly to what we would expect from the polar cap current system, thus con-

firming the importance of that system in understanding polar magnetic

variations.

Discussion

The rather specific interaction between the interplanetary mag-

netic field and the polar geomagnetic field discussed in the present

paper could give important clues to improved physical understanding of

the interaction processes between these two fields.

24.

Wilhjelm and Friis-Christensen (1972) and JgSrgensen et al.

(1972) have suggested that an electric field, which is induced in the

magnetosphere by merging of interplanetary and geomagnetic field lines,

is transmitted to the ionosphere along equipotentials of the polar cusp

field lines; the electric field vector being determined by the azimuthal

component of IMF. This mechanism produces zonal currents along short

segments of approximately equal invariant latitude but fails to explain

the closed polar cap current system reported in the present study.

Bassolo et al. (1972) have sought an explanation for the sec-

tor polarity effects in a mechanism involving interaction between the

radial component of the IMF and the geomagnetospheric tail field. But

since the azimuthal IMF component seems to be the effective one, the

mechanism proposed by Bassolo et al. is probably not applicable.

Heppner (1972a,b) has discovered a dawn-dusk asymmetry related

to the IMF polarity in the polar cap electric fields observed by satel-

lite. When the earth is within an IMF away polarity sector a maximum

polar cap electric field is observed on the evening side at southern

high latitudes. When the earth is within a toward sector the situation

is reversed. Interpreting the electric field measurements entirely in

terms of the convection velocity of geomagnetic field lines across the

polar cap implies, as pointed out by Heppner, that fast convection

occurs where the field lines of the IMF and the field direction in the

outermost regions of the magnetosphere are parallel and slow convection

occurs on the side with anti-parallel field lines. Since one would

expect that the transfer of momentum, or convective velocity, from the

solar wind to the geomagnetic field is most effective in the case of

25.

anti-parallel field lines, this is an indication that a process differ-

ent from convection contributes to the dawn-to-dusk component of the

polar cap electric field. Also the shifts of the polar cap-auroral belt

boundary proposed by Heppner (1972b) to explain the observed influence

of the IMF polarity on the polar cap magnetic field do not reproduce the

more detailed pattern of magnetic perturbations reported in the present

paper.

Stern (1972) has discussed the possibility that unipolar in-

duction between the magnetosphere and the moving magnetosheath may feed

energy from the solar wind into the magnetosphere and high-latitude

ionosphere explaining the correlation of the azimuthal component of IMF

with polar electric fields and with polar magnetic variations. Stern

considers unipolar induction in a circuit configuration where a current

from the magnetosheath enters the polar cusps along the field lines,

then flows across the polar cap in the ionosphere and from the nightside

of the polar cap completes the circuit along tail field lines and the

plasma sheet to the magnetosheath. For the opposite IMF polarity, the

current flow is reversed. It has been pointed out (Stern, personal

communication) that this theory encounters some serious problems. In

particular, if the IMF parts smoothly to allow the magnetosphere to

pass through it the two polar cusps will intersect two neighboring

interplanetary field lines and will be effectively short-circuited to

each other; in addition, no external flux is cut by the magnetosphere

in this case, so that no unipolar effect arises from its motion relative

to the solar wind.

. Kawasaki and Akasofu (1972) propose that the so-called DP-2

26.

variations (Nishida and Kokubun, 1971) and the variations described in

the present paper are related, and that a single current system or

mechanism is responsible for both types of variations. Furthermore

they tentatively conclude that all variations in the polar cap are

associated with polar magnetic substorms. The present study does not

support this view and we are still left with the problem of a new and

unexpected polar cap current system.

At present no satisfactory theory exists for the very direct

and specific solar-terrestrial relationship connecting the polar cap

geomagnetic field to the azimuthal component of the interplanetary

field.

Acknowledgements

The author thanks John M. Wilcox for stimulating discussions

and comments, and also David P. Stern, Koji Kawasaki and Knud Lassen for

their comments on a draft of this paper. This work was supported in

part by the Office of Naval Research under Contract N00014-67-A-0112-0068,

by the National Aeronautics and Space Administration under Grant NGR

05-020-559, and by the Atmospheric Sciences Section, National Science

Foundation under Grant GA-31138.

27.

References

Bassolo, V. S., S. M. Mansurov, and V. P. Shabansky, On a possible cause

of the north-south asymmetry in geomagnetic variations at high

latitudes (in Russian), Issledovanija po Geomagnetismu,

Aeronomii i Fizika Solntsa, 23, 125-131, 1972.

Friis-Christensen, E., The influence of the direction of the interplane-

tary magnetic field on the morphology of geomagnetic varia-

tions at high magnetic latitudes, Geophys. Papers R-27, 149 pp.,

Dan. Meteorol. Inst., Copenhagen, 1971.

Friis-Christensen, E., K. Lassen, J. M. Wilcox, W. Gonzalez, and D. S.

Colburn, Interplanetary magnetic sector polarity from polar

geomagnetic field observations, Nature Physical Science, 233,

48-50, 1971.

Friis-Christensen, E., K. Lassen, J. Wilhjelm, J. M. Wilcox, W. Gonzalez,

and D. S. Colburn, Critical component of the interplanetary

magnetic field responsible for large geomagnetic effects in

the polar cap, J. Geophys. Res., 77, 3371-3376, 1972.

Heppner, J. P., High-latitude electric fields, in Proceedings of the

Symposium on Critical Problems of Magnetospheric Physics,

National Academy of Sciences, Washington, D. C., in press,

1972a.

Heppner, J. P. , Polar-cap electric field distributions related to the

interplanetary magnetic field direction, J. Geophys. Res., 77,

4877-4887, 1972b.

J^rgensen, T. S., E. Friis-Christensen, and J. Wilhjelm, Interplanetary

magnetic-field direction and high-latitude ionospheric

28.

currents, J. Geophys. Res., 77, 1976-1977, 1972.

Kawasaki, K. and S.-I. Akasofu, A study of polar cap and auroral zone

magnetic variations, Report UAG-18, World Data Center A for

Solar-Terrestrial Physics, NOAA, Boulder, Colorado, 21 pp.,

June 1972.

Mansurov, S. M., New evidence of a relationship between magnetic fields

in space and on earth, Geomagnetism and Aeronomy, 9, 622-623,

1969.

Mansurov, S. M. and L. G. Mansurova, Relationship between the magnetic

fields of space and of the earth, Geomagnetism and Aeronomy,

11, 92-94, 1971a.

Mansurov, S. M. and L. G. Mansurova, Annual geomagnetic field variations

in the polar caps, Geomagnetism and Aeronomy, 11, 560-563,

197Ib.

Nishida, A. and S. Kokubun, New polar magnetic disturbances: S , SP,

DPC and DP-2, Rev. Geophys. and Space Phys., 9, 417-425, 1971.

Nishida, A., S. Kokubun, and N. Iwasaki, Annual variation in the magneto-

spheric configuration, and its influence on the polar magnetic

field, Rep. lonosph. Space Res. Japan, 20, 73-78, 1966.

Rosenberg, R. L. and P. J. Coleman, Jr., Heliographic latitude dependence

of the dominant polarity of the interplanetary magnetic field,

J. Geophys. Res., 74, 5611-5622, 1969.

Stern, D. P., Unipolar induction in the magnetosphere, Rep. X-641-72-133,

25 pp., Goddard Space Flight Center, Greenbelt, Md., May 1972.

Svalgaard, L., Sector structure of the interplanetary magnetic field and

the daily variation of the geomagnetic field at high latitudes,

29.

Geophys. Papers R-6, 11 pp., Dan. Meteorol. Inst., Copenhagen,

August 1968.

Svalgaard, L., Interplanetary magnetic sector structure 1926-1971,

J. Geophys. Res., 77, 4027-4034, 1972.

Wilcox, J. M., The interplanetary magnetic field. Solar origin and

terrestrial effects, Space Science Reviews, 8, 258-328, 1968.

Wilcox, J. M. , Inferring the interplanetary magnetic field by observing

the polar geomagnetic field, Reviews of Geophysics and Space

Physics, 10(4), in press, 1972.

Wilcox, J. M. and P. H. Scherrer, Annual and solar-magnetic-cycle vari-

ations in the interplanetary magnetic field, 1926-1971,

J. Geophys. Res., 77(28), in press, 1972.

Wilhjelm, J. and E. Friis-Christensen, Electric fields and high-latitude

zonal currents induced by merging of field lines, Geophys.

Papers R-31, 17 pp., Dan. Meteorol. Inst., Copenhagen,

December 1971, Planet. Space Sci., 20, in press, 1972.

30.

200/

100

VARIATION OF X AND Y ATRESOLUTE BAY (summer)

Figure 1. Average dailyvariation of

geomagnetic field com-ponents in the polar cap.

(a) Variation of X and Yat Resolute Bay during thesummer season of 1965. Thesolid circles show the ob-served X variation and thefull line the smoothed Xvariation. The brokencurve shows the smootheddaily variation of the Ycomponent.

12 18 24 UT

12

UT HOURFOR

o SUMMER• WINTER

AT R M1 1 I

6i »12 UT HI

(b) Local time control ofthe time of observed maxi-mum value of the northcomponent for four northernpolar cap stations.

R FORLOCAL MIDNIGHT

60/

AT

40

20

_Q 9_0.o

I I I iAT R M

»l.7± I.8/IN SUMMER

•23.6±l.6yIN WINTER

(c) Uniformity of theamplitude of the daily var-iation of the horizontalcomponents. The approxi-mate invariant latitude forthe four stations used isgiven below;

A Alert

T Thule

R Resolute Bay

M Mould Bay

86?4

86.2

83.7

81.2

STATIONS

31.

zy700

680

660

640

620

57500

+20

AZy

-20

6UT

6UT

Figure 2. Daily variation of the vertical component at Resolute Bay for1965. Open circles show the variation,qn_day_s where, the earth

was immersed in interplanetary sectors of magnetic field pointing awayfrom the sun. Filled circles show the variation during towards sectors.The broken curve is the average variation on all days irrespective of thesector polarity, and the straight broken line indicates the location ofthe quiet undisturbed level. The average variation has been subtractedfrom the data in the lower panel of the figure.

32.

or*

-20-

Figure 3. Daily variation of the North component at Resolute Bay for1965. The format is the same as used in Figure 2.

33.

GPo

A'̂ -̂ f

POLAR CAP DISTURBANCES AT I8h UT FOUNDDURING IMF AWAY POLARITY.

>A CIRCULATING IONOSPHERIC HALL CURRENTFLOWING EASTWARD AROUND THE MAGNETIC

POLE (MP) FITS THE OBSERVATIONS.

Figure 4. Synoptic map of polar cap magnetic perturbations at 18 UTduring IMF away polarity. Horizontal perturbations are shown

as vectors attached to the station circles for Alert (AL) ~ Thule (TH), 'Resolute Bay (RB), Mould Bay (MLB), Godhavn (GO), and Baker Lake (BL).Vertical perturbations are given as signed numbers next to the stationcircles. A circulating current which may produce the magnetic pertur-bations is indicated in the figure.

34.

GPe

BLf-,0

SCALE:

AZy

POLAR CAP DISTURBANCES AT I8h UT FOUNDDURING IMF TOWARD POLARITY.

A CIRCULATING IONOSPHERIC HALL CURRENTFLOWING WESTWARD AROUND THE MAGNETIC

POLE (MP) FITS THE OBSERVATIONS.

Figure 5. Same as Figure 4, but for IMF toward polarity. The directionof all perturbations is reversed compared to the situation

found for away sectors, and the current encircling the pole is alsoreversed. The positions of the geographical pole (GP) and of the invari-ant magnetic pole (MP) are shown in both figures.

35.

AZr10

-10

^ TOWARD POLARITY'

\

INVARIANT60° LATITUDE

AWAY POLARITY :o

|AH|X

10

n

•/'o\

/ \• \^ i

~- / \

/ & 9

1 1 1

80 70(INVARIANT

60o LATITUDE

YEARLY AVERAGES OF POLAR CAP DISTURBANCESHOWN SEPARATELY FOR EACH POLARITYOF THE INTERPLANETARY MAGNETIC FIELD.

Figure 6. Difference between the yearly mean of Z and H for both IMF -polarities and the all-day average values (see text). The

differences are plotted against invariant latitude for nine stations athigh northern latitudes. Open circles are used for away polarity andfilled circles for toward polarity.

36.

AWAY

(o)

'5<""24\t 2I>

• |

30RESOLUTE BAY

AY

r

TOWARDAZ=49/AT I7.0h

30

TOWARDAZ= 24/AT I9.0h

-R AT I8.3r

(b)

21 AWAY

AT 19.5 h

30

MOULD BAY

(c)

Figure 7. Daily rotation of horizontal perturbation vectors for (a) Thule,(b) Resolute Bay, and (c) Mould Bay. Successive endpoints of

the vectors are marked by the corresponding UT values at three-hour inter-vals. The vectors tend to stay within the same quadrant for each IMFpolarity. The time and amplitude for maximum vertical perturbation, AZ,is shown for each polarity for the three stations, as is the sense ofrotation of the horizontal perturbation vectors. -R means clockwise rota-tion and +R means counterclockwise vector rotation, as viewed from abovethe pole.

37.

100

AZr 50

0

• RESOLUTE® THULEO ALERT

BAY

W WSEASON

MAGNITUDE OF THE POLARITYEFFECT IN THE POLAR CAP.

Figure 8. Seasonal variation of the magnitude of the vertical perturba-tions caused by the polar cap current (see text).

38.

UT

17 -

16 -

15 -

U -

13 -

12 -

11 -

10

09 -

08 -

WINTER

SUMMER

16.2LocalNoon

Alert (1964)

J F M A M J J A S O N D J F M A M J J

8.0^Magnetic"Noon

UT14

13

12

11

10

09

08

07

06

05

Monthly mean time of maximal Z-per turbat ionat Aler t during 1964.

agneticoon

SUMMER

WINTER

Vostok (1959-61)

.Local

Noon

i i i i i i i i i i i i i i i i i i rJ F M A M J J A S O N D ' J F M A M J J

Monthly mean time of maximal Z-per turbat ionat Vostok during 1959-61.

Figure 9. Monthly mean time (modal value) of the maximal Z-perturbationon days with inferred towards IMF polarity for Alert in the

northern polar cap, and on days with inferred away polarity for Vostokin the southern polar cap for the years shown.

39.

80W

PROPOSED NEARLY UNIFORM SEASONAL VARIATION OFMAGNETIC ELEMENTS NEAR THE NORTH MAGNETIC POLE.

THE DIFFERENCES BETWEEN SUMMER AND WINTER ARESHOWN FOR THE HORIZONTAL AND VERTICAL COMPONENTS.

AVERAGE HORIZONTAL DIFFERENCE IS 14 y POINTINGNORTHWARDS. THE AVERAGE VERTICAL DIFFERENCE ISI7y POINTING UPWARDS.

"Figure 10. Synoptic map of systematic differences between summer andwinter mean values of the magnetic elements for the

northern polar cap during 1965 (see text).

40.

SEASONAL VARIATION OF MAGNETIC ELEMENTS1965

ALERT Ax

THULE

RESOLUTEBAY

MOULDBAY

GODHAVN

20 /

AY

* oW E S E W W E S E W

20r

AX

0

W E S E W

/*\

W E S E W

or

AZ

-40

or

Az-40

or

AzS -40

W E S E W

W E S E W

VW E S E W

orAz

-40

W E S E W

20 y

AX

W E S E W

°rAz

-8 -40

E W

•\o

9--,

W E S E W

Figure 11. Seasonal variation of the magnetic elements in the northernpolar cap during 1965 shown separately for IMF away polarity

(open circles), toward polarity (filled circles), and for all days in eachseason (broken curves). W, E, and S mean winter, equinoctial, and summerseason respectively.

41.

sowGEOMAGNETIC PERTURBATIONS DURING AWAY SECTORSWITHIN 120 DAYS CENTERED ON SUMMER SOLSTICE

PERTURBATIONAMPLITUDE

DISTANCEFROM CENTER

-20/

POSITIVE IMF POLARITY

Figure 12. Synoptic map of the difference between the mean values of themagnetic elements for days within IMF away sectors and the

all-day seasonal average for the summer 1965 over the northern polar cap.The format is similar to that in Figures 4 and 5. The.amplitude of theperturbations as function .of invariant latitude is shown in the lowerpanel. Compare with Figure 6.

42.

80W

GEOMAGNETIC PERTURBATIONS DURING TOWARDS SECTORSWITHIN 120 DAYS CENTERED ON SUMMER SOLSTICE

20/

PERTURBATIONAMPLITUDE

DISTANCEFROM CENTER

20 x

NEGATIVE IMF POLARITY

Figure 13. Same as Figure 12, but for days within towards sectors.

43.

UNCLASSIFIEDSecurity Classification

DOCUMENT CONTROL DATA - R & DfSeeurily classification ot title, body ol abstract and indexing annotation must be entered when the overall report is classilied)

1. ORIGINATING ACT IVIT Y (Corporate author)

Institute for Plasma ResearchStanford UniversityStanford, California 94305

2fl. REPORT SECURITY CLASSIFICATION

UNCLASSIFIED26. GROUP

3. REPORT TITLE

POLAR CAP MAGNETIC VARIATIONS AND THEIR RELATIONSHIP WITH THE INTERPLANETARYMAGNETIC SECTOR STRUCTURE

4. DESCRIPTIVE NOTES (Type ol report end inclusive dates)

Scientific Interim5. A U T H O R ( S ) (First name, middle initial, last name)

Leif Svalgaard

6. REPORT DATE

^n Qon-t-emVioT1 1Q79

8a, CONTRACT OR GRANT NO.

N00014-67-A-0112-0068 >b. PROJECT NO.

NR 323-003c.

d.

^a. TOTAL NO. OF PAGES 76. NO. OF REFS

44 21

9a. ORIGINATOR'S REPORT NUMBER(S)

SUIPR Report No. 486

9b. OTHER REPORT NO'S) (Any other numbers that may be assignedthis report)

10. DISTRIBUTION STATEMENT

This document has been approved for public release and sale; its distributionis unlimited.

11. SUPPLEMENTARY NOTES

TECH. OTHER

12. SPONSORING MILITARY ACTIVITY

Office of Naval Research800 North Quincy StreetArlington, Virginia 22217

13. ABSTRACTThe relationship between polar geomagnetic variations and the polarity of the

interplanetary magnetic sectors has been studied for the quiet year 1965. It isfound that during the day-hours a system of ionospheric currents encircles the mag-netic poles on every day. The current system may extend up to 15 from the pole,but is strongest at 8-10 invariant colatitude. The current direction as seen byan observer standing on the earth is counterclockwise during interplanetary sectorswith field pointing away from the sun, and clockwise during toward sectors. Thecurrent strength is dependent on season, being strongest during local summer. Whenthe magnetic pole is on the nightside of the earth, this polar cap current is absentor very weak. When the rotation of the earth brings the magnetic pole into thedayside, the polar cap current system develops, with the current being most concen-trated in the part of the current system which is nearest to the noon meridian. Thecurrent increases its total intensity until the magnetic pole is rotated past thenoon meridian; then the intensity decreases as the magnetic pole approaches thenightside again. The seasonal variation of the magnetic elements in the polar capis discussed in view of the sector polarity effects. These effects introduce an im-portant modulation of the seasonal variations of the geomagnetic polar field. Thedemonstration of current systems inside the polar caps encircling the magnetic polesduring local day hours calls for a major revision of the generally accepted pictureof polar cap geomagnetic variations. It also suggests a new framework for inter-preting polar cap observations of geomagnetic and related phenomena.

DD FORMi MOV as1473 UNCLASSIFIED

Security Classification

UNCLASSIFIEDSecurity Classification

14.

KEY WORDS

POLAR CAP MAGNETIC VARIATIONS

INTERPLANETARY MAGNETIC SECTOR STRUCTURE

IONOSPHERIC CURRENT SYSTEM

L I N K A

ROLE WT

L I N K B

ROLE WT

L I N K C

ROLE WT

UNCLASSIFIEDSecurity Classification